1. Field of the Invention
The present invention relates to a liquid crystal projector in which three liquid crystal panels corresponding to separated three red, green and blue light beams are used.
2. Description Related to the Prior Art
The liquid crystal projector projects a light through a projection lens to display an image on a screen with enlarging a small image displayed on a liquid crystal panel. As types of liquid crystal projector for displaying a color image, there are a single panel type in which only one liquid crystal panel is provided, and a three-panel type in which three liquid crystal panels are provided. The single panel type has merits in that the device has a simple structure and is cheap and small. However, an efficiency of using a light beam is small, the bright image is hardly displayed. Further, as the one liquid crystal panel is provided with three liquid crystal cells respectively corresponding to three RGB colors (R; red, G; green, B; blue). Accordingly, it is hard to display the image with higher fineness. In the three-panel type, there are demerits in the higher price than the single panel type, and in the larger size of the device. However, as three liquid crystal panels respectively corresponding to the RGB colors are used, the device can display the image with higher fineness.
As the liquid crystal projector of three panel type, U.S. Pat. No. 6,046,858 (Corresponding to Japanese Patent Publication No. 2001-520403) discloses a system for displaying a full-color image, which is provided with a color separating optical system, three light valves (liquid crystal panel of reflection type), a color composing member and a projecting lens. In the light separating optical system a white light beam of linear polarization is separated into light beams of RGB colors. The three light valves of reflecting type respectively receive the RGB light beams and perform the modulation thereof at each pixel depending on the image to be displayed. Then the modulated light beams exits from the pixels of three light valves. The color composing member composes the modulated light beams into one projection light, which is projected with enlargement through the projecting lens to form an image on a screen.
However, in the above liquid crystal projector, a cross-dichroic prism is used as the color composing member, and in order to increase an efficiency of using the light, an incident green light is previously polarized to a p-polarized light, and incident red and blue lights are previously polarized to an s-polarized light. In this case, according to the characteristics of the dichroic mirrors, the p-polarized green light travels through the cross-dichroic prism, and the s-polarized incident red and blue lights reflect on the cross dichroic mirrors. Thereafter, the three S-, P-, and S-polarized lights are composed, which is named SPS composition.
In order to perform the SPS composition, as shown in FIG. 6, a ½ wavelength plate 132 is provided between a polarizing plate 114G and the reflection type light valve 131G for converting the polarizing direction of the light incident into the cross-dichroic prism. In the white dot display, each liquid crystal light valve reflects the incident light with the rotation of the direction of the polarization. In the black dot display, each liquid crystal light valve reflects the incident light without the rotation of the direction of the polarization.
The color combining optical system asks of the incident lights “SPS” characteristics for its better efficiency. Then, the incident lights to the reflection type liquid crystal display must be “PSP” characteristics. The outgoing lights from the color separating optical system have “SSS” characteristics. So, in the blue and the red light path, a ½ wavelength plate is needed between the color separating optical system and the liquid crystal display. However, as the ½ wavelength plate is provided, the number of parts becomes larger, and the assembling cost becomes higher. Therefore the producing cost becomes larger.